diff --git a/Examples/GreetingService/Package.swift b/Examples/GreetingService/Package.swift
index 5561f560..660ab19e 100644
--- a/Examples/GreetingService/Package.swift
+++ b/Examples/GreetingService/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:5.8
+// swift-tools-version:5.9
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftOpenAPIGenerator open source project
diff --git a/IntegrationTest/Package.swift b/IntegrationTest/Package.swift
index 46027467..0c8f3560 100644
--- a/IntegrationTest/Package.swift
+++ b/IntegrationTest/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:5.8
+// swift-tools-version:5.9
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftOpenAPIGenerator open source project
diff --git a/Package.swift b/Package.swift
index 21c01d71..26d34819 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:5.8
+// swift-tools-version:5.9
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftOpenAPIGenerator open source project
@@ -16,14 +16,11 @@ import Foundation
 import PackageDescription
 
 // General Swift-settings for all targets.
-var swiftSettings: [SwiftSetting] = []
-
-#if swift(>=5.9)
-swiftSettings.append(
+var swiftSettings: [SwiftSetting] = [
     // https://github.com/apple/swift-evolution/blob/main/proposals/0335-existential-any.md
     // Require `any` for existential types.
     .enableUpcomingFeature("ExistentialAny")
-)
+]
 
 // Strict concurrency is enabled in CI; use this environment variable to enable it locally.
 if ProcessInfo.processInfo.environment["SWIFT_OPENAPI_STRICT_CONCURRENCY"].flatMap(Bool.init) ?? false {
@@ -31,7 +28,6 @@ if ProcessInfo.processInfo.environment["SWIFT_OPENAPI_STRICT_CONCURRENCY"].flatM
         .define("SWIFT_OPENAPI_STRICT_CONCURRENCY"), .enableExperimentalFeature("StrictConcurrency"),
     ])
 }
-#endif
 
 let package = Package(
     name: "swift-openapi-generator",
diff --git a/README.md b/README.md
index 406e0588..b330a007 100644
--- a/README.md
+++ b/README.md
@@ -36,7 +36,8 @@ Choose one of the transports listed below, or create your own by adopting the `C
 | Generator versions | Supported OpenAPI versions | Minimum Swift version |
 | -------- | ------- | ----- |
 | `0.1.0` ... `0.1.11` | 3.0 | 5.8 |
-| `0.1.12` ... `main` | 3.0, 3.1 | 5.8 |
+| `0.1.12` ... `0.3.5` | 3.0, 3.1 | 5.8 |
+| `1.0.0-alpha.1` ... `main` | 3.0, 3.1 | 5.9 |
 
 ### Supported platforms and minimum versions
 
diff --git a/Sources/swift-openapi-generator/Documentation.docc/Swift-OpenAPI-Generator.md b/Sources/swift-openapi-generator/Documentation.docc/Swift-OpenAPI-Generator.md
index bfa9d757..e36abbaa 100644
--- a/Sources/swift-openapi-generator/Documentation.docc/Swift-OpenAPI-Generator.md
+++ b/Sources/swift-openapi-generator/Documentation.docc/Swift-OpenAPI-Generator.md
@@ -44,7 +44,8 @@ Choose one of the transports listed below, or create your own by adopting the `C
 | Generator versions | Supported OpenAPI versions | Minimum Swift version |
 | -------- | ------- | ----- |
 | `0.1.0` ... `0.1.11` | 3.0 | 5.8 |
-| `0.1.12` ... `main` | 3.0, 3.1 | 5.8 |
+| `0.1.12` ... `0.3.5` | 3.0, 3.1 | 5.8 |
+| `1.0.0-alpha.1` ... `main` | 3.0, 3.1 | 5.9 |
 
 See also <doc:Supported-OpenAPI-features>.
 
diff --git a/Tests/OpenAPIGeneratorReferenceTests/CompatabilityTest.swift b/Tests/OpenAPIGeneratorReferenceTests/CompatabilityTest.swift
index 3603247d..64bb6a23 100644
--- a/Tests/OpenAPIGeneratorReferenceTests/CompatabilityTest.swift
+++ b/Tests/OpenAPIGeneratorReferenceTests/CompatabilityTest.swift
@@ -259,7 +259,7 @@ fileprivate extension CompatibilityTest {
             XCTAssertNoThrow(try FileManager.default.createDirectory(at: packageDir, withIntermediateDirectories: true))
             let packageSwiftPath = packageDir.appendingPathComponent("Package.swift", isDirectory: false)
             let packageSwiftContents = """
-                // swift-tools-version:5.8
+                // swift-tools-version:5.9
                 import PackageDescription
                 let package = Package(
                     name: "\(packageName)",
diff --git a/docker/Dockerfile b/docker/Dockerfile
index 76bb652d..5040ad9c 100644
--- a/docker/Dockerfile
+++ b/docker/Dockerfile
@@ -1,4 +1,4 @@
-ARG swift_version=5.8
+ARG swift_version=5.9
 ARG ubuntu_version=jammy
 ARG base_image=swift:${swift_version}-${ubuntu_version}
 
diff --git a/docker/docker-compose.2204.58.yaml b/docker/docker-compose.2204.58.yaml
deleted file mode 100644
index b2b4c827..00000000
--- a/docker/docker-compose.2204.58.yaml
+++ /dev/null
@@ -1,18 +0,0 @@
-version: "3"
-
-services:
-  runtime-setup:
-    image: &image swift-openapi-generator:22.04-5.8
-    build:
-      args:
-        ubuntu_version: "jammy"
-        swift_version: "5.8"
-
-  test:
-    image: *image
-    environment:
-      - WARN_AS_ERROR_ARG=-Xswiftc -warnings-as-errors
-      - IMPORT_CHECK_ARG=--explicit-target-dependency-import-check error
-      - STRICT_CONCURRENCY_ARG=-Xswiftc -strict-concurrency=complete
-  shell:
-    image: *image
diff --git a/docker/docker-compose.yaml b/docker/docker-compose.yaml
index 35b5d820..48f5cd7d 100644
--- a/docker/docker-compose.yaml
+++ b/docker/docker-compose.yaml
@@ -4,7 +4,7 @@
 #
 # % docker-compose \
 #   -f docker/docker-compose.yaml \
-#   -f docker/docker-compose.2204.58.yaml \
+#   -f docker/docker-compose.2204.59.yaml \
 #   run test
 #
 version: "3"